Fosphenytoin sodium, a phosphate ester prodrug of phenytoin, was developed as a replacement for parenteral phenytoin sodium. Unlike phenytoin, fosphenytoin is freely soluble in aqueous solutions, including standard i.v. solutions, and is rapidly absorbed by the i.m. route. Fosphenytoin is metabolized (conversion half-life of 8 to 15 min) to phenytoin by endogenous phosphatases. Therapeutic free (unbound) and total plasma phenytoin concentrations are consistently attained after i.m. or i.v. administration of fosphenytoin loading doses. Fosphenytoin has fewer local adverse effects (e.g., pain, burning, and itching at the injection site) after i.m. or i.v. administration than parenteral phenytoin. Systemic effects related to the CNS are similar for both preparations, but transient paresthesias are more common with fosphenytoin.